Holography is a form of optical information storage. The general principles are described in references such as "Photography by Laser" in SCIENTIFIC AMERICAN, 212 (No. 6), 24-35 (1965). In a holographic process, the object to be photographed or imaged is illuminated with coherent light (e.g., from a laser), and a light sensitive recording medium (e.g., a photographic plate) is positioned to receive light reflected from the object. Each point on the object reflects light to the entire recording medium, and each point on the medium receives light from the entire object. The reflected light beam is known as the object beam. Simultaneously a portion of the coherent light is beamed by a mirror directly to the medium, bypassing the object. The light is known as the reference beam. What is recorded on the recording medium is the interference pattern that results from the interaction of the reference beam and the object beam impinging on the medium. When the processed recording medium is subsequently illuminated and observed, the light from the illuminating source is diffracted by the hologram to reproduce the wave-front that originally reached the medium from the object. The hologram resembles a window through which the virtual image of the object is observed in full three-dimensional form, complete with parallax.
Holograms that are formed by allowing the reference and object beams to enter the recording medium from the same side are known as transmission holograms. Interaction of the object and reference beams in the recording medium forms fringe areas with varying refractive indices which are normal or near normal to the plane of the recording medium. When the hologram is played back by viewing with transmitted light, these fringes refract the light to produce the viewed virtual image. Such transmission holograms may be produced by well-known methods such as those described in U.S. Pat. Nos. 3,506,327; 3,694,218; 3,838,903; 3,894,787; 3,951,663; 4,055,423; 4,139,388; 4,258,111; and 4,374,189.
A diffraction grating is the simplest form of transmission hologram. It is the hologram of two coherent plane waves. It can be created by splitting a single laser beam and recombining the beams at the recording medium.
A diffraction grating can be characterized by its Diffraction Efficiency, i.e., the percent of the incident radiation which is diffracted into forming the image. The greater the Diffraction Efficiency, the brighter the image in viewing light.
Refractive index modulation is a quantitative measure of the change in refractive index image. For the diffraction grating, refractive index modulation is the measure of the amplitude of the sinusoidal modulation of the refractive index within the recording medium produced when the holographic image is recorded. The refractive index modulation, or index modulation, for a volume recording medium is best determined by holographically forming a grating in the medium and calculating the index modulation using Kogelnik's coupled wave theory and the measured parameters of the grating formed, i.e, the Diffraction Efficiency, medium thickness, and the like.
A variety of materials have been used to record volume holograms, such as silver halide emulsions, hardened dichromated gelatin, ferroelectric crystals, photopolymers, photochromics and photodichroics. Characteristics of these materials are given in Volume Holography and Volume Gratings. Academic Press, New York, 1981, chapter 10, pp 254-304 by L. Solymar and D. J. Cook.
Dichromated gelatin is the material most widely used for recording volume holograms. This material has become the popular choice because of its high Diffraction Efficiency and low noise characteristics. However, the material has poor shelf life and requires wet processing to develop the hologram. Wet processing involves an additional step in hologram preparation, and also can cause the hologram image to distort due to volume changes in the gelatin medium during processing. Reproducibility is difficult to achieve with dichromated gelatin.
U.S. Pat. No. 3,658,526 describes solid photopolymerizable materials that require only a single process step to prepare stable high-resolution holograms with a refractive index image. The refractive index modulation is in the range of 0.001-0.003, and reconstructed holographic images are reported to have limited brightness because of low Diffraction Efficiency.
There is continuing interest in the development of new and improved holographic media, and methods for single step holographic recording of optical information.
Accordingly, it is an object of this invention to provide polymeric film media which can record a refractive index modulation pattern induced by a laser interference light pattern.
It is a further object of this invention to provide a single step process for recording a phase holographic image in a polymeric thin film by means of a laser interference pattern induced molecular rearrangement.
Other objects and advantages of the present invention will become apparent from the accompanying description and examples.